Mechanical isolation tools, such as isolation plugs, have been used for many years in the pipeline industry to facilitate pipeline isolation, for example, where no valves have been provided in a given section of pipe.
These plugs are typically “piggable,” that is, they can be transported into and through the pipeline using fluid pressure in the pipe and use a combination of taper locks to secure the plug within the pipe and either hydraulic set compression or inflation seals to engage the pipe wall to permit isolation of the pipe. Some plugs operate a fail-safe mechanism whereby the plug is configured so that differential fluid pressure acting across the plug maintains the seal and/or the lock in an energized configuration independently of the set pressure used to actuate the plug. Such plugs are described as being self energized.
A number of plug designs seek to provide double block and bleed isolation whereby two axially spaced seals engage the pipe wall, with the region between the seals being arranged to be vented or bled to facilitate testing or monitoring of the seals or pipe integrity. Generally, such plug designs fall within two categories.
In a first arrangement, two seals are provided with an annulus void between them and are retained by a single array of taper lock segments. This configuration has the advantage of providing a bleed facility while retaining fail safe energization on both seals.
In a second arrangement, a secondary array of locks is added between the seals and the plug is configured as two separate modules connected together by a bend compliant joint, each module containing a single seal and lock array. This configuration offers added redundancy in the addition of a second lock array, but has the limitation of requiring trapped pressure between the seals in order to provide fail-safe self energization of the secondary seal. This trapped pressure could be generated by high pressure hydrocarbons which are then isolated from the worksite by a single seal. Existing plug designs also have two lock and seal modules connected with couplings which provide only angular movement. As a result, only one lock array may be tested.
A number of variations have been made to the designs described above. For example, each of the seal and lock units may consist of separate modules for ease of assembly and transportation though the pipe.